An optical parametric amplifier (OPA) is a laser-based light source that emits light tunable in wavelength generated by a nonlinear optical process. In this processes high-intensity laser output is converted to light of wavelengths (or equivalently frequencies) different from that of the laser using suitable nonlinear optical materials. The nonlinear materials are typically crystals that exhibit nonlinear optical properties. This process of wavelength conversion is dependent on the relative phases of the waves involved and without proper phase matching the converted light intensity oscillates at insignificant small levels. Phase matching typically can be obtained over a narrow spectral region, allowing conversion in that region to grow. The narrow spectral region of phase matching can be used for frequency selection. Introducing a low intensity pulse at the desired conversion frequency within the spectral bandwidth of phase matching and synchronized to the pump pulse is referred to as “seeding”. Seeding improves the spectral and spatial quality of the tunable light generated in the conversion process.
Three pulses or waves present in an OPA are called the pump, signal and idler. The pump pulse is typically the output of a laser. The pump, at frequency ωp, provides energy for amplification of the signal and generation of an idler pulse. The signal and idler frequencies are less than that of the pump and are called ωs and ωi respectively. The relationship ωp=ωs+ωi holds between the three frequencies. This process can be thought of as one photon of an incident laser pulse (pump) being divided into two lower-energy photons (signal and idler) by a nonlinear optical crystal. The relationship of frequencies can also be expressed in terms of the free-space wavelengths of the pump, signal and idler as 1/λp=1/λs+1/λi.
An OPA has incident light at the signal frequency called the seed along with the higher intensity pump light. An optical parametric generator (OPG) has only incident pump light and no incident seed. The signal and idler grow from random background fluctuations in an OPG. The output beams of an OPG are usually relatively weak and are relatively spread-out in direction and frequency. Some spectral narrowing and reduced divergence can be obtained by using optical parametric amplification following an OPG. If the purpose of the wavelength conversion is the generation of idler light from input of incident pump and signal light, the process may be called difference frequency generation.
The wavelengths of the signal and the idler waves are determined by the phase matching condition, which can be changed by temperature or, in bulk birefringent nonlinear crystals by the angle between the incident pump laser ray and the optical axes of the crystal. In a method called quasi-phase matching (QPM), the orientation of the nonlinear crystal is changed after each coherence length so that the sign of the effective nonlinear coefficient is reversed. A coherence length is the distance over which energy conversion flows from the pump to the signal and idler in a uniform nonlinear crystal. Back conversion occurs as the propagation distance exceeds the coherence length, and signal and idler energies are converted back to pump energy. Changing the sign of the nonlinear coefficient after each domain that is of a length that is an odd-integer multiple of the coherence length allows the conversion of pump to signal and idler to continue. The coherence length is determined by the three OPA wavelengths and dispersion of the nonlinear material. A domain is a region of uniform crystal orientation. A quasi-phase-matching period comprises two domains of alternating crystal orientation.
Periodically poled lithium niobate (PPLN) is an example of a quasi-phase-matched nonlinear material. The period of the poling is usually two coherence lengths, a domain of one coherence length followed by a second domain of inverted orientation also of one coherence length. This structure is repeated periodically throughout the length of the quasi-phase-matched material. Typical periods for the wavelengths of examples presented in this Disclosure are approximately 30 microns. The QPM period determines the wavelengths of phase matching on large or coarse scale, and temperature tuning is commonly used to finely adjust the wavelengths of phase matching. The QPM period can also be slowly increased or decreased (chirped) over the length of the crystal. Chirped QPM has been shown to be useful with ultrashort optical pulses when differences in group velocity cause different spectral components of the pulses to overlap at different positions in the nonlinear crystal.
Group velocities describe the propagation speed of the individual pulses, and differ significantly for the pump, signal, and idler ultrashort pulses. Group velocity is determined by dispersion of an optical material. With ultrashort pulses, normal differences in group velocity can cause the signal or the idler pulse to completely propagate through a pump pulse in millimeter crystal lengths. Propagation paths with dispersion controlled by prisms or optical gratings are commonly used to expand ultra short pulses to longer pulses with a frequency chirp or compress frequency-chirped longer pulses to shorter pulses with no frequency chirp. With the use of ultra-short pulses or expanded pulses with frequency chirp, synchronization of a seed pulse with the pump pulse in an OPA is important.
Photonic crystal fibers (PCFs) have a regular structure that is constant over the length of the fiber. An example referenced herein is a silica glass fiber with an array of circular voids arranged in a hexagonal pattern with the central void absent. An all normal dispersion (ANDi) fiber has group velocity only increasing with increasing wavelength in the spectral region of concern. Structure in a wave such as modulation propagates with the group velocity. Dispersion in an optical fiber is determined both by the bulk dispersion of the material from which the fiber is made and the structure of the fiber. The diameter and spacing of the voids contribute to the dispersive properties of the fiber.
A super continuum (SC) pulse is generated by the action of a nonlinear index of refraction and group velocity dispersion. As pulse intensity increases the nonlinear index increases causing slower propagation. The leading slope of the pulse with increasing intensity is expanded in time and the trailing slope of the pulse is compressed in time. The local wavelength is increased in the leading slope and decrease on the trailing slope in a process called self-phase modulation (SPM). With normal group velocity dispersion the leading-slope, longer-wavelength portion propagates faster, and the trailing-slope, shorter-wavelength portion propagates more slowly. Wavelength-shifted light in the slopes of a pulse can be advanced or delayed to overlap un-shifted light in the tails of the pulse. In an effect called optical wave breaking (OWB) the overlapping pulse portions of different wavelength interact to produce wavelength side bands further broadening the pulse. The result is a SC pulse expanded in wavelength and time.